In a synchronous telecommunications network or system, such as a Synchronous Digital Hierarchy (SDH) or SONET transmission network, traffic data is transported across the network via a transmission path. A path may comprise a number of sections, each section comprising a transport service between two adjacent nodes of the network. Each network node typically comprises a synchronous transmission apparatus, such as a multiplexer, a regenerator or a cross-connect, for receiving the traffic and, where applicable, processing the traffic and transmitting it on to another network node or to a local terminal.
It is common to provide path protection such that the traffic arrives at a destination node via two independent paths namely, a main path and a stand-by path. If, for any reason, the main path should be compromised or should fail, then the stand-by path is selected. Thus, at the destination node, a synchronous transmission apparatus is provided with a respective port for each incoming path and a single port for output of the selected path. The transmission quality of each path is evaluated at the respective ingress port and the results are communicated to the output port. A quality comparison is made at the output port and the path of higher quality is selected for output to the traffic destination. A protection switching apparatus is normally used to switch between the incoming data paths depending on which is selected.
A problem with conventional protection switching is that there is a traffic hit, i.e. an interruption in the traffic stream, whenever a protection switch is performed, irrespective of whether the switch is made as a result of, say, a planned maintenance activity or an equipment or optical fibre failure. A main cause of traffic hits during protection switching results from a differential time delay between the traffic arriving on the respective main and stand-by data paths. The differential time delay can arise for a variety of reasons including the respective physical distances of each data path, and the number and nature of intermediate network apparatus on each path.
Network operator-to-customer and network operator-to-network operator quality of service contracts typically specify required error performances for traffic transmission. In today's networks, a traffic outage of 50 ms during protection switching is considered to be a maximum tolerable level. Failure to meet specified error targets can result in loss of revenue or even loss of contract for a network operator. Hence, there is a need to minimize or eliminate traffic hits resulting from protection switching.